This application relates to copending patent application filed on even date herewith entitled xe2x80x9cAIRBORNE SATELLITE COMMUNICATION SYSTEM HAVING A SATELLITE SELECTION ALGORITHM THEREIN WHICH IS DEPENDENT UPON AN ANTENNA MOUNTING CHARACTERISTIC AND AN ANGULAR DISTANCE BETWEEN AN ANTENNA NORMAL LINE AND A LINE TO A SATELLITExe2x80x9d by the same inventor and assigned to the same assignee.
The present invention generally relates to airborne satellite voice and data communication systems. In recent years, airlines have increased the availability of satellite voice and data communication systems on board commercial aircraft. For many airlines, satellite communication systems are preferred because of the nearly worldwide coverage area that is frequently available. For example, communication satellites from INMARSAT cover almost the entire globe. Both flight deck and cabin voice and data communications are now available on this worldwide system known in the industry as AERO H, for the high gain transmitter/antenna system used to communicate in the global satellite beams. INMARSAT has offered three types of services for its airline customers: AERO H, which provides a worldwide coverage area with a mosaic of xe2x80x9cglobal beamsxe2x80x9d, and a more geographically limited service known as AERO I, which uses intermediate gain transmitter/antenna systems. The intermediate gain transmitters communicate only while in xe2x80x9cspot beamsxe2x80x9d or nearby areas where the satellites are able to receive signals from the lower power transmitters/antenna systems. The AERO H transmitter antenna system would typically transmit in excess of 40 watts, while the typical AERO I transmitter/antenna system transmits in a range of approximately 20 to 25 watts. The final INMARSAT aeronautical service, AERO L, is a data only service that makes use of the global beam, much in the same manner of AERO H.
The development of spot beam technology allows for use of a smaller, less expensive transmitter and smaller and less expensive antenna systems. Typically, these transmitters have multiple channels allowing several (typically 5) simultaneous conversations from the aircraft to terrestrial positions while the aircraft is within a satellite""s spot beam. To prevent one transmitter from interfering with another, satellite service providers require emitted signals from terminals to meet minimum spectral purity requirements that tend to artificially limit the amount of power a transmitter may emit. While the AERO I transmitter/antenna systems have some very significant advantages, such as reduced cost, size and weight, it also has drawbacks as well.
One major drawback of the AERO I system, in comparison with the AERO H system, is its inability to provide truly worldwide communication ability. With their lower power transmitters, AERO I systems typically are only able to provide normal communication services while within or relatively close to spot beams. While this may be economically efficient for many airlines, it is often undesirable for flight crews, especially during times of emergencies in remote areas of the world.
Consequently, there exists a need for improvement in satellite communication systems which provides low cost, low weight and smaller sized transmitter/antenna systems while concomitantly providing expanded emergency communication service areas.
It is an object of the present invention to increase service coverage for lower power transmitter/antenna systems.
It is a feature of the present invention to include a transmitter having a relatively high power transmitter while in emergency communications mode.
It is an advantage of the present invention to provide nearly worldwide emergency service from a system normally operating only within limited spot beam areas.
It is another object of the present invention to use a lower power transmitter in conjunction with a lower gain antenna system.
It is another feature of the present invention to include a dual mode transmitter where one of the modes is a single channel mode for use during emergencies only.
It is an advantage of the present invention to avoid the distortion products that would otherwise be associated with simultaneous multi-channel operations on multiple frequencies at increased power levels.
The present invention is a method and apparatus for providing nearly worldwide emergency service on a multi-channel satellite voice and data communication system which is typically limited to operation only in spot beams, which is designed to satisfy the aforementioned needs, to achieve the above-included objects, include the previously stated features, and arrive at the already articulated advantages.
The present invention is carried out in a xe2x80x9cdistortion product-lessxe2x80x9d system in a sense that the interference that would otherwise result from the distortion products of multiple channels operating simultaneously through a lower power transmitter are eliminated while operating at higher power levels.
Accordingly, the present invention is an apparatus and method which includes a multi-mode transmitter where one mode is an emergency mode in which all but one of the multiple channels are disabled, and the single remaining channel is allowed to broadcast at a higher power level than when the apparatus is operated in another mode, the multi-channel mode.